DP Versus FDL


Thread Starter

Filipe Campos

Hi. What is the principal difference between profibus FDL and S7 ? In what cases shall use S7 instead of FDL? If I have a PLC with a lot's of data to report to a Command Center, which is the best protocol? regards, Filipe Campos

Michael Griffin

This isn't the sort of question which can be answered completely in a short letter. Siemens offers all of their manuals free on their web site as PDF files, I suggest that for a detailed answer (or as detailed as you are going to get), you down load a few. However, I will mention a few things. Your subject is titled "DP Versus FDL", but in your message you asked about "FDL and S7". These are not the same thing. DP is a form of remote I/O and is used to poll for small blocks of data repeatedly. FDL and "S7 communications" (as Siemens describes it) are intended for higher level communications between PLCs, or between PLCs and PCs and allows these systems to exchange larger blocks of data less frequently (this is perhaps an over simplification). FDL is a subset of FMS, and is what is found in the S5 series (or at least the S5-95U). "S7 communications" is what was implemented on the new S7-300 and 400 (but not the 200) series. It apparently is another subset of FMS, but the exact difference between FDL and "S7" appears to be some sort of secret (or perhaps Siemens has confused themselves as badly as they are confusing everyone else). What you need to use depends upon what model of PLC you have. ********************** Michael Griffin London, Ont. Canada [email protected] **********************

Rob Hulsebos

>What is the principal difference between >profibus FDL and S7 ? S7 is the name for a family of PLC's, and FDL is a part of the Profibus family of protocols. Although I have never actually verified this myself, I've been told that 1) the MPI protocol is a variation of FDL, and 2) that the S7 functionblocks SEND and RECEIVE use FDL instead of the other available Profibus protocols (FMS, DP) for the simple reason that the functionality needed for these two functionblocks is not available in FMS or DP. It wouldn't be difficult to verify this if I had two S7's and a Profibus protocol analyzer, shouldn't take more than a few minutes. The implementation of these functionblocks might change as there is now also an IEC 61131 profile for DP, available from the Profibus users group (if you are a member). Rob
> What is the principal difference between profibus FDL and S7 ? < FDL is the data link layer (layer 2) of the OSI model for Profibus. It is used BY ALL Profibus protocols (here is where I take an exception to the gentleman that stated FDL is a subset of FMS). As a layer 2 protocol, the data delivered by FDL has no defined upper layers; make it what you want... S7 is a fully defined (to the application layer), connection based protocol defined and OWNED by Siemens. Therefore your client application can ask for PLC addresses etc... FDL - efficient, no definition for the upper layers..you must define S7 - defined, proprietary, LOTS of overhead..well defined application layer for your client applications.